feat(panel): display multiple account cells

[Phototonic]

Summary

• Add an independent per-account Show this account in the panel setting while keeping the existing active-account selection and popup behavior unchanged.
• Render marked accounts as compact, side-by-side COSMIC panel cells with deterministic ordering, resize handling, migration, and lifecycle pruning.
• Refresh marked accounts safely alongside selected accounts, with batch ownership and request handling that preserve the active selection across concurrent outcomes.

Validation

• cargo fmt
• cargo test
• just check
• git diff --check
• Manual standalone-dev check: marked accounts rendered in the simulated panel and panel visibility toggles were verified on and off.

Scope

• No popup aggregation or per-account panel-style overrides.
• The active-account radio selection remains separate from panel visibility.